The present invention relates to an electrically driven power steering control apparatus, and more specifically relates to a protecting apparatus that is capable of restricting a current flowing in a motor to prevent the motor and its control circuit from being excessively heated, thereby protecting the motor and its control circuit against heat deterioration or thermal failure.
An electrically driven power steering apparatus includes a motor that generates a steering assist torque to be added to a steering mechanism. To avoid the burnout of a motor, the magnitude of drive current supplied to a motor should be restricted. For example, there is a method for restricting a drive current supplied to a motor to suppress excessive heating or burnout of the motor. In this case, a current restricting operation begins when a predetermined time has passed after starting the steering operation, and then the motor drive current decreases along a predetermined curve.
According to this method, from the view point of suppressing motor burnout, in a case that a driver continues the steering operation until the current restricting operation begins, it was necessary to greatly reduce the drive current supplied to the motor upon starting the current restricting operation. Thus, it was necessary to set a large change width (i.e. reduction rate) of the steering assist torque relative to elapse of time.
According to the above-described conventional electrically driven power steering apparatus, a motor burnout preventing mechanism starts its operation when a driver must manage to put a vehicle into a garage or into a limited parking space by repetitively turning a steering wheel for a relatively long time. The steering assist torque abruptly decreases and accordingly a steering torque required for a driver increases rapidly within a relatively short period of time. Such an abrupt increase in the steering torque will force a driver to feel sudden change in the reaction force returning from the steering wheel or feel abnormal behavior in the steering wheel.
To this end, there is a conventionally known electrically driven power steering apparatus that discriminates a motor component having a small thermal time constant from a motor component having a large thermal time constant and sets a limit current value for each of these motor components. This apparatus restricts the drive current supplied to a motor with reference to a lower limit current value being thus determined and accordingly can prevent a control circuit or the motor from being excessively heated. It becomes possible to surely protect the motor components having the tendency of easily causing the burnout due to their shorter thermal time constants as well as other motor components having longer thermal time constants. A restoration time of the motor current becomes short. In general, an excessive current protection decreases a steering assist force. However, this apparatus prevents the steering feeling from being worsened due to such reduction in the steering assist force (refer to Japanese Patent Application Laid-open No. 2002-78379).
Furthermore, there is another conventional technique according to which the steering assist torque is continuously maintained by using a means for reducing a power current corresponding to a steering torque so as to supply a requisite minimum level of current, thereby eliminating sudden drop in the steering assist torque (refer to Japanese Patent Application Laid-open No. 2001-341661).
The system disclosed in the patent document 1 requires a judgment as to whether or not the drive current supplied to a motor should be restricted. This judgment is substantially dependent on a thermal time constant (i.e. a thermal endurance) of a used constituent component. Furthermore, according to the system disclosed in the patent document 2, the power current corresponding to a steering torque changes in accordance with an allowable current feedable to the motor. Accordingly, both systems disclosed in the patent documents 1 and 2 can prevent sudden drop in the steering assist torque. However, these conventional systems must determine the power current according to constituent components or control conditions. Accordingly, it was difficult to completely eliminate strange feeling during a steering operation made by a driver.
Furthermore, according to the system disclosed in the patent document 1, it is necessary to revise the limit current value for each new constituent component employed for an electrically driven power steering apparatus when this component has a different thermal time constant. Furthermore, the control program for the electrically driven power steering apparatus must be revised. In other words, it is difficult to use a common control program for numerous types of electrically driven power steering apparatuses. The costs required in the research and development stage will increase.
According to the system disclosed in the patent document 2, the allowable current feedable to a motor is dependent on the characteristics of each motor or its peripheral circuit. Thus, it is necessary to reevaluate the allowable current for each motor or its peripheral circuit used in an electrically driven power steering apparatus. Furthermore, the control program for the electrically driven power steering apparatus must be revised. Thus, the costs required in the research and development stage will increase.